"Our research would suggest that restricting sunbathing or visits to the tanning booth to morning hours would reduce the risk of skin cancer in humans," says Sancar, and colleague Professor Sarah Graham Kenan from the UNC School of Medicine.

"However, further studies in humans are needed before we can make any definitive recommendations."

Skin cancer is the most common form of cancer in Australia and the United States, and is mainly caused by DNA damage as a result of UV light exposure.

UV damage is repaired by an enzyme in the skin called xeroderma pigmentosum group A (XPA) protein, which is active in both mice and humans.

Sancar and colleagues have found that in mice, XPA levels change throughout the day with lowest levels at 4am in the morning and highest levels at 4pm in the afternoon.

When experimental mice were irradiated with UV in the morning, they were five times more likely to develop tumours than when they were irradiated in the afternoon, the researchers report.

Mice, like humans, are on a 24-hour circadian clock, but unlike humans they are nocturnal so their clock runs counter to the human clock.

Sancar and colleagues predict that in humans, XPA levels are therefore likely to be highest in the morning and lowest in the afternoon.

They now plan to measure actual DNA repair rates in the skin of human volunteers to test the hypothesis that morning sun presents less of a skin cancer risk than afternoon sun.

But he says even if it applies in humans, it might be hard to generalise, because humans are known to have a range of circadian rhythms with some being "night owls" and some "morning larks".

Professor Ian Olver, Chief Executive Officer of Cancer Council of Australia describes the idea that circadian rhythms alter cancer risk as "very interesting" but says it is unclear how it will affect public health advice if human studies do find a similar link.

Olver says there are 10,000 melanomas in Australia per year and 1830 people deaths from skin cancer.

"We have to be very careful when we change our messages that we don't make those figures any worse," he says.

Olver says it will be important to consider the intensity of UV radiation as well as the body's capacity to repair the damage it causes.

But, says Ulver, recent research suggests that UV A light can do more damage than previously thought in the morning and afternoon.

To add to the complexity, there are other biochemical triggers for cancer and DNA repair that are yet to be explored. There are also still gaps in our knowledge on the relationship between sunlight and vitamin D production.

"We don't yet quite know precisely how much sunlight you need and at what intensity to produce vitamin D," says Ulver.

But, he adds, "In general, in summer it's worth covering up for most of the day."